Silicate-impregnated polyolefin capacitor

ABSTRACT

Polyolefin film dielectric capacitors are impregnated with a dielectric of a tetraalkyl orthosilicate to yield high corona starting voltage capacitors.

United States Patent [50] 317/258, 259, 260;2S2/63.7; 336/58, 94; 174/17LF Raynor Linzey, Stamford, Conn. 009,7 13

[56] References Cited UNITED STATES PATENTS Appl. No. [22] Filed [45]Patented Feb. 9, 1970 Nnv. 2, 1971 [73] Assignee Sprague ElectricCompany North Adams, Mas.

Attorneys-Connolly and Hutz, Vincent 1-1. Sweeney, James 54SILICATEJMPREGNATED POLYOLEFIN Paul O'Sullivan and David R. ThorntonCAPACITOR 6 Claims, 1 Drawing Fig.

ABSTRACT: Polyolefin film dielectric capacitors are impregnated with adielectric of a tetraalkyl orthosilicate to yield high corona startingvo1tage capacitors.

H L m c a s n U I 1 .1 2 l U 5 SILICATE-IMPREGNATED POLYOLEFIN CAPACITORBACKGROUND OF THE INVENTION The present invention is concerned withelectrical capacitors having high corona starting voltagecharacteristics.

Recently polyolefin film capacitors e.g., polypropylene, have beenreported to be capable of being fully impregnated with a dielectricliquid of the type comprising a halogenated aromatic compound havingfrom 1 to 5 chlorine substituents and from 1 to 3 aryl groups. It ismaintained that corona starting voltage tests on units of this type haveindicated that the liquid dielectric has completely impregnated all ofthe capaci tor including the polyolefin film. It has been determined,however, that it is only with extreme difficulty that this type of unitcan be impregnated, even when employing a paper dielectric film as awicking aid in conjunction with the polypropylene film. As anillustration of the difficulty involved in fully impregnatingpolypropylene with a commercially available halogenated aromaticdielectric fluid, it has taken anywhere from one to three weeks at about90 C., to cause the polypropylene film to takeup about 11 percent byweight of the liquid while at the same time dissolving about l percentby weight of the film in the dielectric liquid.

In view of this difficulty, it has been found necessary, for theimpregnation of commercial units, to employ with the polypropylene film,a porous cellulosic dielectric, e.g., Kraft paper, and also to employspecial heat cycling procedures, after initial impregnation of the unit,in order to achieve maximum impregnation of the device. Such meansobviously increase the cost of the unit. It would be a considerableadvance in the art to be able to employ polypropylene film as the solefilm dielectric in a capacitor and also to employ a fluid dielectricwhich can easily and completely impregnate the unit.

It is an object of the invention to present a capacitor capable ofoperation at higher voltage stresses having high volumetric efficiency.

Another object of the invention is to provide an electrical capacitorhaving extremely high corona starting voltage characteristics.

It is yet another object of the present invention to provide an allpolyolefin film dielectric capacitor characterized by case of fluiddielectric impregnation.

Other objects and advantages of the present invention will be madeobvious to those skilled in the art by the following description whenconsidered in relation to the accompanying drawing ofwhich: the soleFIGURE shows a partially unrolled convolutely wound capacitor of thetype employed in the present invention.

SUMMARY OF THE INVENTION In accordance with the present invention thereis provided a capacitance section sealed within a capacitor housingmeans. The capacitance section comprises at least a pair of electrodesand a polyolefin dielectric spacer between the electrodes. The capacitoris fully impregnated with at least a major proportion of a tetraalkylorthosilicate. The alkyl groups of the orthosilicate are either straightor branched chain and preferably have from six to 18 carbon atomstherein.

In another embodiment of the invention the tetraalkyl orthosilicatedielectric may contain up to 45 percent by weight of conventional fluiddielectrics, for example, a fluid halogenated aromatic compound havingfrom 1-5 chlorine substituents and from 1-3 aryl groups, mineral oil,cottonseed oil, silicone oil, etc.

In another embodiment of the invention a conventional porous filmdielectric of the cellulosic type e.g., Kraft paper, may be employed inconjunction with the polyolefin film. This film will function as a wickwhich will lead in the fluid dielectric to the interior of the unit andfacilitate its contact with the polyolefin film.

BRIEF DESCRIPTION OF THE DRAWING Other objects and advantages of thepresent invention will be made obvious to those skilled in the art bythe following description when considered in relation to theaccompanying drawing of which:

The sole FIGURE shows a partially unrolled convolutely wound capacitorof the type employed in the present invention.

The drawing illustrates a capacitance section 10 of the type with whichthe present invention is concerned. This section will be positionedwithin a capacitor container or housing (not shown) and laterimpregnated with the fluid dielectric of the present invention. Section10 comprises a convolutely wound section having metal electrodes 11 and12 separated by polyolefin films 13 and 14. The capacitance section haselectrode tabs 15 and 16 which are in contact with electrodes 11 and 12and which will thereafter make contact to the under side of aconventional capacitor can cover assembly, containing tab terminals.

DETAILED DESCRIPTION OF THE INVENTION It has been determined, withrespect to dielectric fluid impregnated AC capacitors, that there is aclose correlation between the degree of impregnation and the coronastart voltage (CSV) of the unit. When the corona start voltageapproaches a maximum or is comparatively high, it has been found thatthere has also been a substantially complete impregnation of the unit.On the other hand when the corona start voltage is low, it has beenfound that there has been poor or incomplete liquid impregnation of theunit. The following example illustrates the preparation of capacitors ofthe present invention and the corona start voltage data indicates thatthe units have been completely or at least substantially completelyimpregnated.

EXAMPLE Four, 0.2 microfarad, convolutely wound capacitors were preparedemploying a single thickness of 0.5 mil polypropylene film separating0.25 mil aluminum foils. The units were placed in a conventional metalcontainer and closed with terminal-containing can covers also ofconventional design. The covers contain a small fill-hole therein. Thecapacitor units were vacuum dried and impregnated through the fill-holewith tetra-Z-ethylhexyl orthosilicate at a temperature of about C. overa period of about one-half hour. The fill-hole was sealed and the unitswere not subjected to the common practice of a heat cycle to enhanceimpregnation of the units.

The units were then tested according to standard procedures for coronastarting voltage. In three of the four units, the capacitance sectionsbroke down (Et 3,000

VAC/mil) before corona start. In the fourth unit corona started at 3,800VAC/mil.

By way of comparison, identical units, impregnated in the same manner,but with a commercially available halogenated aromatic dielectric fluid,revealed a corona starting voltage of from about 800-l,100 VAC/mil. Thecomparatively high breakdown and high corona starting voltage of theunits of the present invention indicate that substantially completefluid dielectric impregnation had been obtained. On the other hand, thelow corona starting voltage of the comparison units indicate that theywere not fully impregnated with the halogenated aromatic fluiddielectric.

The ease of silicate impregnation of polypropylene was further confirmedby simple beaker tests which established that after one-half hour at 90'C., a film of 0.5 mil polypropylene absorbed about 6 percent by weightof tetra-2- ethylhexyl orthosilicate. The film thickness increased byabout 10 percent. Prolonging the test beyond one-half hour did notincrease the amount of silicate absorbed nor did it increase thethickness of the f lm.

The polyolefin films contemplated by the present invention includepolypropylene, polyethylene, polystyrene, etc. It is preferred to employisotactic polypropylene of fairly high purity. This type ofpolypropylene is presently commercially available from several differentsources. For some purposes it is advisable to employ in conjunction withthe polyolefin film a porous cellulosic type spacer to assist inimpregnation and to act as a wick for the fluid impregnant. A commonlyemployed material for this purpose is Kraft paper.

While the specific example herein has shown the use oftetra-2-ethylhexyl orthosilicate as the impregnant, it is to beunderstood that other silicates defined by the general formula (R),Sican be employed. Within this broad definition the R group can be analkyl group having from six to 18 carbon atoms in straight or branchedchain configuration. The R groups of the silicate may be all the same ormay or may be different alkyl groups. Typical examples of the alkylgroups are 2-ethylbutyl, 2-ethylhexyl, 1,3-dimethylbutyl,2-methylpentyl, l-ethylpentyl, l-methylhexyl, etc. It is also to beunderstood that the alkyl groups of the silicate may contain aromatic orchlorine-containing aromatic substituents.

Also, while the specific example has shown the use of an orthosilicateas the sole impregnant, it is to be understood that the contemplatedsilicates may be combined or blended with other compatible prior artdielectric impregnants in order to obtain specific advantages. Forexample, in order to improve the impregnation characteristics of acommercially available chlorinated hydrocarbon such as Clorinol (amixture of chlorinated diphenyls containing about 42 percent by weightchlorine), up to about 45 percent by volume of this material may beblended with the orthosilicate. This will impart to the Clorinol theadvantages of the silicate while at the same time taking advantage ofthe low cost of the Clorinol. The use of a chlorinated hydrocarbon aspart of the liquid impregnant will also impart fire proofingcharacteristics to the impregnant. In addition to chlorinatedhydrocarbons, other commonly used liquid impregnants such as mineraloil, silicone oil, cottonseed oil, etc., may also be employed.*Registered Trademark Sprague Electric Company Where only nonchlorinecontaining silicates are employed herein there is no need to add priorart epoxy type stabilizers since there is no possibility of generatinghydrochloric acid to be scavenged. Where either chlorine-containingsilicates or silicates mixed with chlorine-containing liquid dielectricsare employed certain epoxy type stabilizers such as dipentene dioxide,l-epoxyethyl, 3,4-epoxycyclohexane, etc., may be added to these liquidimpregnants. While aluminum has been illustrated as one electrodematerial useful in the present invention other metal foils, for example,tin, lead, silver, etc., may also be employed. It is also contemplatedthat one or both of the capacitor electrodes may be in the form of athin metallization on the surface of the polyolefin film. An example ofthis form is aluminized polypropylene.

Since it is obvious that many changes and modifications can be made inthe above described details without departing from the nature and spiritof the invention, it is to be understood that the invention is notlimited to such details except as set forth in the appended claims.

What is claimed is:

1. An electrical capacitor comprising a capacitance section having atleast a pair of electrodes convolutely wound in contact with at leastone polyolefin dielectric spacer between said electrodes, said sectionbeing fully impregnated with a liquid dielectric consisting essentiallyof a tetraalkyl orthosilicate, wherein the alkyl group is straight orbranched chain and has from six to 18 carbon atoms.

2. The capacitor of claim 1 wherein said polyolefin is polypropylene.

3. The capacitor of claim 2 wherein said liquid dielectric istetra-2-ethylhexyl orthosilicate.

4. The capacitor of claim 2 wherein said liquid dielectric istetra-Z-ethylbut lorthosiljcate.

5. The capacl or of claim 1 wherein at least one of sald electrodes isin the form of a metallization layer deposited on a surface of saidpolyolefin spacer.

6. The capacitor of claim 5 wherein said metallization is aluminum.

l i i

2. The capacitor of claim 1 wherein said polyolefin is polypropylene. 3.The capacitor of claim 2 wherein said liquid dielectric istetra-2-ethylhexyl orthosilicate.
 4. The capacitor of claim 2 whereinsaid liquid dielectric is tetra-2-ethylbutylorthosilicate.
 5. Thecapacitor of claim 1 wherein at least one of said electrodes is in theform of a metallization layer deposited on a surface of said polyolefinspacer.
 6. The capacitor of claim 5 wherein said metallization isaluminum.